The invention relates generally to superconducting magnet systems and, more particularly, to an integrated coil winding concept within a superconducting magnet system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In one example, an MR system comprises a superconducting magnet, a magnet coil support structure, and a helium vessel. Liquid helium contained in the helium vessel provides cooling for the superconducting magnet and maintains the superconducting magnet at a low temperature for superconducting operations, as will be understood by those skilled in the art. The liquid helium maintains the superconducting magnet approximately and/or substantially at the liquid helium temperature of 4.2 Kelvin (K). For thermal isolation, in one example, the helium vessel that contains the liquid helium comprises a pressure vessel inside a vacuum vessel.
An MR superconducting magnet typically includes several coils, a set of primary coils that produce a uniform B0 field at the imaging volume, and a set of bucking coils that limit the fringe field of the magnet. These coils are wound with superconductors such as NbTi or Nb3Sn conductors. The magnet is cooled down to liquid helium temperature (4.2 K) so that the conductors are operated at their superconducting state. The heat loads of the magnet, such as that produced by the radiation and conduction from the environment, are removed by either the boil-off of liquid helium in an “open system” or by a 4 K cryocooler in a “closed system”. The magnet is typically placed in a cryostat to minimize its heat loads since the replacement of liquid helium is expensive and since the cooling power of a cryocooler is limited.
When the several coils of the superconducting magnet are not physically symmetric about a mid-plane axis, field homogeneity can suffer. Furthermore, electrically coupling coil pairs unsymmetrically about the mid-plane axis can cause a net magnetic flux coupling, F, with the z-gradient coil, especially if the z-gradient coil is not fully shielded or it is unshielded. A z-gradient pulse can generate electromotive forces in each turn of the magnet coils (e=−dΦ/dt). The electromotive forces can accumulate in the magnet coils, which can result in induced currents or induced voltages in the coils, depending on the magnet coil circuit. The induced current would negatively affect the homogeneity and stability of the B0 field in the imaging volume. The induced voltage can damage the coil insulation and may induce partial discharges (PD) as well. The partial discharges can cause insulation aging and negatively affect the imaging quality.
It would therefore be desirable to have an apparatus and method capable of providing magnetic field homogeneity, avoiding induced currents and voltages, and ensuring the electric insulation of the magnet coils under gradient pulses.